1. Field of the Invention
This invention relates to a photomask management method used in a photolithography process when a semiconductor integrated circuit device is manufactured and a photomask wash limit generating method, and more particularly to a photomask management system using the above methods.
2. Description of the Related Art
Recently, when a semiconductor integrated circuit device, for example, a semiconductor memory device is manufactured, the integration density of interconnects and elements configuring a circuit is enhanced and a pattern is increasingly miniaturized. In order to form a pattern on a wafer by exposing a photomask in photolithography, it is said necessary to use KrF light up to the generation of 0.09 μm and ArF light after the generation of 0.07 μm as exposure light.
However, in a case where photomasks are used in a semiconductor manufacturing facility, growth defects occur on a photomask of the ArF generation if it is kept used. As a result, an increase in the line width or a line-to-line short occurs in a transfer pattern and is detected in a wafer defect checking step, thus causing a problem that the device yield is lowered.
Further, the growth defects occurring on the photomask can be eliminated by washing the photomask and the washed photomask can be returned to and reused in the semiconductor manufacturing facility (for example, refer to Jpn. Pat. Appln. KOKAI Publication No. 2004-40038). However, since damages are caused in a halftone (HT) film by washing the photomask, the transmittance and phase difference are varied to reduce the exposure margin.
Therefore, when photomasks are first shipped from the mask manufacturing process to the photolithography process for device manufacturing (in an unused state), photomasks in which the phase differences and transmittances of the films are already largely deviated from ideal values contain masks which cannot be washed any more, that is, which cannot be used any more even if they are washed although they satisfy the specification.
In the conventional flow of eliminating the growth defects, first, a desired mask pattern is drawn in a mask manufacturing process. Then, a development step and etching step are performed to form a photomask pattern. Next, a photomask washing step is performed. After this, a defect checking step is performed with desired checking sensitivity. If it is detected in the checking step that foreign matter is attached (the photomask is determined to be defective), the checking step is performed again after the washing step is performed. Then, the washing step and defect checking step are repeatedly performed until the foreign matter is removed (the photomask is determined to be serviceable).
After this, an exposure margin is calculated based on an optical measurement result and dimension measurement result acquired from a measurement database according to the optical measurement and dimension measurement procedures in the mask manufacturing process.
Specifically, the transmittance and the phase difference of a halftone film of a desired portion are measured in the optical measurement step. Then, the dimension measurement of a desired pattern is made in the dimension measurement step. The optical measurement and dimension measurement data items are stored in the measurement database. The measurements are transferred to a determining system which determines the flexible specification of the present mask and an exposure (lithography) margin is calculated in a desired optical condition.
In this case, the exposure (lithography) margin thus calculated is compared with a permissible exposure (lithography) margin previously defined to determine whether the mask is serviceable or defective.
If the determination result indicates that the mask is defective, a mask starts to be manufactured starting from the mask drawing step again. If the determination result indicates that the mask is serviceable, a defect checking step is performed. If it is detected in the checking step that foreign matter is attached (defective), the washing step is performed again and then the checking step is performed again. After this, the above routine is repeated until the process to the checking step after the optical measurement step, dimension measurement step and flexible specification determining step are performed is determined to be serviceable.
In the above checking step, a photomask which is determined to be serviceable is transferred to a photolithography process for device manufacturing after a pellicle attaching step. After the photomask is transferred to the photolithography process for device manufacturing, first, an exposure condition is determined by use of a lithography margin management pattern and an adequate exposure value is acquired.
Further, an exposure step is performed by use of the above adequate exposure value and a defect checking step (mask contamination confirmation step) is performed by use of a desired number of days or exposure processes. If the checking result indicates a serviceable mask, an exposure step is performed by use of the adequate exposure value, a defect checking step (mask contamination confirmation step) is performed again by use of a desired number of days or exposure processes and the above routine is repeated until the checking result indicates a defective mask (foreign matter detected).
If the checking result indicates a defective mask, the photomask is transferred to the mask manufacturing process. First, a pellicle is separated from the photomask and the washing step is performed. Further, the checking step, optical measurement step, dimension measurement step and flexible specification determining step are performed. After this, a photomask which is determined to be serviceable is transferred again to the photolithography process for device manufacturing after the checking step and pellicle attaching step are performed and then the above process is repeated.
However, in the case of the conventional photomask checking method, although a lithography margin is not ensured for a borderline pattern in which the lithography tolerance is small when a photomask is actually used in the flexible specification determining step in the mask manufacturing process, it is erroneously determined that the photomask can be used and the yield is lowered in the device manufacturing process in some cases. On the other hand, although a lithography margin is ensured for a borderline pattern in which the lithography tolerance is small, it is erroneously determined that the photomask cannot be used and there occurs a possibility that the photomask is wastefully discarded (for example, refer to Japanese Patent Specification No. 3425414).
Further, only the quality determination is made in the flexible specification determining step. Therefore, when the photomask is used in the photolithography process for device manufacturing, information as to how many times the mask can be washed, that is, the number of washing steps which can be further performed cannot be acquired. As a result, it is difficult to determine whether or not the mask washing step can be performed after the mask is once contaminated, that is, whether or not the mask can be still used even after the mask washing step is performed.
When the contaminated mask is washed in the mask manufacturing process after the photolithography process for device manufacturing while the above determination cannot be made, an optical measurement result and dimension measurement result are acquired and then a flexible determination step is performed. However, if the photomask should be determined to be defective in the flexible determination step, it becomes necessary to form a mask again. In this case, there occurs a problem that it takes a long time to return the photomask to the photolithography process for device manufacturing and, as a result, a device product manufacturing process using the photomask in the photolithography process for device manufacturing is delayed.